WASHINGTON (AP) — The U.S. Secret Service says it has intercepted a letter addressed to President Barack Obama that contained a “suspicious substance.”

Also, one of the Michigan offices of long-time Democratic senator, Carl Levin, has been evacuated after a suspicious letter was received late this morning, The Associated Press reports.

The FBI says preliminary tests on a letter sent to President Barack Obama indicate the presence of poisonous ricin.

The letter is undergoing further testing because preliminary field tests can be unreliable, creating false positives.

The letter was intercepted at a facility away from the White House. It comes the day after officials said a letter sent to Sen. Roger Wicker tested positive for poisonous ricin. That letter to Wicker, a Republican, was intercepted at a Senate mail facility just outside Washington.

The FBI says there is no indication of a connection to the bombing at Monday’s Boston Marathon.

Secret Service spokesman Ed Donovan says the letter was intercepted at a facility away from the White House. He says the letter was received Tuesday.

The letter comes a day after lawmakers said a letter was mailed to Sen. Roger Wicker that tested positive for poisonous ricin. Another senator said police have a suspect in mind.

Tensions have been high in Washington and across the country since the deadly bombings on Monday at the Boston Marathon that killed three people and injured more than 170.

Michigan Sen. Carl Levin says his regional office in Saginaw received a suspicious letter and that authorities have been alerted.

Levin says in a statement that an aide received the letter Wednesday, but did not open it. Authorities are now investigating.

The Democratic lawmaker says he and his staff do not know if the mail presented a threat.

The FBI says those letters are related and are both postmarked out of Memphis, Tenn., dated April 8. Both letters were intercepted at off-site mail facilities.